1. Field of the Invention
The present invention relates to a device for positioning a rotary actuator.
2. Summary of the Prior Art
Many forms of rotary actuator have only two stable positions, corresponding to the opposite ends of the rotary stroke of the actuator. For example, in a known form of pneumatic rotary actuator, compressed air is supplied to one side or the other of a pivoting vane sealed within a housing, to switch the vane between two end stops limiting its travel. Such an arrangement provides an extremely reliable manner of actuating for mechanisms that have only two operating positions, such as a two port valve.
However, there are many mechanisms that rotary actuators can be usefully used to control, but which have three or more operating positions. For instance, three port valves might require three stop positions and two port valves are sometimes used to crudely control flow rate by introducing an intermediate stop position. Accordingly, it is known to control a rotary actuator to stop at a position intermediate its two end positions. In the known pneumatic actuator referred to above this is achieved, for example, by using solenoid valves to control the flow of air to either side of the vane to position and then hold the vane in an intermediate position by balancing the air pressure on opposite sides of the vane.
Such position control mechanisms are, however, rather complex and do not always provide reliable positioning of the actuator.
It is a general aim of the present invention to provide a more reliable device for accurately and consistently positioning a rotary actuator at an intermediate position in its stroke.
Accordingly, there is proposed a device for positioning a rotary actuator, the device comprising a rotatably mounted coupling shaft, one end of which can be coupled to an output of the rotary actuator, and a pair of resilient biassing means for applying oppositely directed torques to the coupling shaft, one of the biassing means urging the shaft in a clockwise direction towards a stop position and the other urging the shaft in an anticlockwise direction towards said stop position.
The positioning device can be coupled to the actuator with the stop position, to which the coupling shaft of the device is urged by both biassing means, aligned with the desired intermediate rotary position of the actuator. In this way, when no other operating force is applied to the actuator, the biassing means act to return the actuator to its intermediate position.
In order to provide a more positive stop position for the coupling shaft, the biassing means preferably act on the shaft through a coupling assembly arranged so that the biassing means urging the coupling shaft in a clockwise direction towards said stop position is uncoupled from the shaft when the shaft rotates in a clockwise direction beyond the stop position and the biassing means urging the coupling shaft in an anticlockwise direction towards said stop position is uncoupled from the shaft when the shaft rotates in an anticlockwise direction beyond the stop position. With this arrangement, even if the biassing means are not balanced with one another, the stop position is accurately defined.
In a particularly preferred form the coupling assembly comprises a pair of coupling members, each of the biassing means acting on the coupling shaft through a respective one of the coupling members. Each coupling member is prevented by a physical stop from rotating beyond the stop position in the direction of the bias, but the coupling shaft is free to continue rotating in this direction independently of the coupling member. In this way, since the biassing means acts on the coupling member, rather than directly on the coupling shaft, once the shaft rotates beyond the stop position, the biassing means ceases to act on it.
The biassing means preferably apply substantially only a torque to the coupling shaft. That is to say, preferably little or no lateral forces are applied to this shaft. Clock-type springs are particularly suitable, since they provide substantially a pure torque output.